Modulation circuit



March 8i, 1938..v D E HARNETT 2,110,664

MODULATION CIRCUIT Filed Aug. 12, 1935 3 Sheets-Sheet 1 y f5v l el y E e f 6" 2 5005er l /7 l y Q ATTQRNEYS March 8, 1938. D. E. HARNETT 2,110,664

MoDULATIoN CIRCUIT Filed Aug. 12.l 1933 5 sheets-'sheet 2 ill'l ATTORNEYS Patented Mar. 8, 193 8 "PATENT ori-lcs f MonULs'rroN omom'r Daniel s. nar-'neta Tucuman. N. y.. misant to s Hazeltine Corporation. a corporation of Dela- WIN Application August iz. 193s. sensi 1ra-escasa,

ze claim. ici. 25osol The present invention relatesto a selective circuit, and more particularly to an adiustably selective system designed to respond simultaneously to two or more widely separated frequeneies.

The system includes appropriately selective. inductively reactive and capacitively reactive elements. so arranged and connected inthe system that adjustment oi a sinsle variable element produces two points ci maximum impedance widely separated and bearing a fixed law of variation with respect to each other as the variable element is adjusted over a substantial range. In its fundamental aspect, the circuit vmay be oonsidered as a tuned circuit having two oscillatory periods widely separated and moving simultane ously as the adjustable element is varied.

It is the object of this invention to produce a circuit which may be tuned to two different frequencies, by means of a singlevvariable element.

It is a further object of this invention to'pro-` duce a system in which a single variable elemment may be utilized to tune said system to simultaneously vary two resonancefrequencies for supplying voltage corresponding to each of the oscillatory periods, to a plurality of vacuum tubes, thus providing a selective input system ior each of said tubes, which system requires a single adjustable clement. v

It is another object of this invention to provide a modulation of frequency-changing circuit for the input of a superheterodyno. which requires a single variable condenser to simultaneously tune to lthe desired signal and oscillation frequency.

These. and further objects of this invention. will become 'apparent from the following specification and claims taken in connection with the accompanying drawings. v

In accomplishing: the object of this invention. with particular yreference to a superheterodyne radio receiver for the purpose of illustration.'the signal frequency tuned input circuit oi. the first 45 `detector or modulator ci the superheterodyne i receiver, is tuned by c. variable condenser in the usual manner to receive audio-modulated carrier waves. The oscillation-frequency-determining v circuit. or oscillation circuit. as it will be referred n" to hereinafter. Vis so connected in shunt across thc signal-frequency tuned circuit. or signal circuit. as it will be referred to hereinafter, that the .same variable condenser may be utilized to tune the oscillation circuit. A high intermediate tre'- 55 qucncy is chosen so that the tuning of each oi the two circuits will have. relatively no ei'lect upon the voltages developed in the other oi'ithe two circuits. In other words, a high-frequency diil'erence is selected so that the tuning inductance of the signal-frequency circuit will act as.

acholre at the oscillation frequency. Similarly,

with a high diilerence, the auxiliary alignment condenser and the oscillation-frequency inductancc will constitute a high impedance in this path at the signal frequency.

Having thus briefly described the invention.

attention is invited to the accompanying drawings in which:

lliig. l is.an elementary circuit diagram of thev fundamental circuit comprising the present invention:

Fig. 2 is a circuitdiasram kof a superheterodyne radio receiver,v including -a frequencychanging circuit,l constructed in accordance with this invention;

Fis. 3 is a diagram of a circuit similar to that of Fig. 2. but'includins an additional radio-frequency ampliller before the irequency-determining circuit; and, Fig isa diagram ci a similar selective circuit. in which a Esingle tube is utilized tc act asY both oscillator and modulator.

Referring now more 'particularly to Fig. .1 of

the drawings. an input circuit connected to a source of high frequency. includes a primary inrductance Li which is inductively coupled to a secondary inductance La, included in the tunable oscillatory circuit il. The circuit i6 may include an additional inductancc La' and variable ccn. denser Cz, by means of which the resonance frequency ci the circuit il may be adiusted. In shunt with the variable condenser Ca is connected the tunable circuit is. which `includes condenser C a, inductance La and condenser C4,

which, together with Ga, constitutes a second tunable oscillatory circuit. The input oi' tubs i! includes the tunable circuit it. and the input of the tube i1 includes the inductance La of the tunable circuit it.

In'operation, the characteristics of the various elemental of tunable circuits it and il are .so chosen that a wide 'frequency difference will exist between the resonance periods of each .of these circuits. the frequency oi.' the tunable circuit Il being higher than that oi' the tunable circuit i4.` At the high resonant-frequency of the circuit il, the total inductance o! circuit I4 will act ,as a high-frequency choke. and at the lower resonantfrequency o! the circuit il, the'capacities C: and C. will actas a high impedance. Furthermore.

nated by the same reference characters.

I the series capacity Caand C4 of circuit I8, act to limit the tuning rangeof the condenserl Cz with respect to the tunable circuit I 8, by means of quencies may be supplied to the input of tube I 5.

However, the/inputv of tube I1 is connected across the inductance La and therefore is substantially isolated from the circuit I4, and consequently,

the tube I1 Iresponds merely to the frequency of the tunable circuit I8. Furthermore, since circuit I4 is parallel-resonant with respect to condenser C2 and circuit I8 is parallel-resonant with respect to inductance La, an impedance maxima will appear across condenser C2 at`the resonant Yfrequency of circuit I4 and across inductance La' at the resonant frequency of circuit I8.

A particular application of this circuit to a superheterodyne radio receiver is illustrated in Fig. 2, in which corresponding elements are desigtance L1.. The inductance L1 is inductively coupled to the inductance La of the signal circuit I4, which circuit is tuned to resonance with the dev sired signal frequency by the variable condenser The' signal frequency voltage developed 1 across the condenser C2 is impressed between the control grid and cathode of the modulator tube I5, which may be of the pentode type as, for instance, the type 58. A gridebiasing resistor R5 is included in the cathode lead of the tube I5 for the purpose of providing the proper grid bias,

and a radio-frequency by-pass condenser C5 is provided in shunt with the biasing resistor.y

. The oscillatorv tube I1, which may be of the triode type, as for instance type 56, has the oscillation circuit I8 connected between its grid and cathode.

the inductance La, the alignment condenser Ca, the variable condenser C2, whichds common to the tuned input circuit I4, and condenser C4.

The oscillation frequency is controlled by the variable condenser C2, the capacity range of which is limited relative to its effect upon the oscillation frequency by the alignment condenser Cs -and to some extent by by-pass condenser C4. A paddingycondenser Cp, for assistingthe alignment, is connected between the junction of inductance Le with condenser C: and ground, and is, therefore, substantially in shunt with the inductance La. The resistor R4 is for th'e purpose of providing the proper bias on the grid vof the oscillator tube I'I. lThe bias is produced by the grid current and limits the oscillation amplitude.

The plate electrode of the tube I1 is connected through the coupling resistor Rzto a. source'of high potential indicated as the battery I6. A radio-frequency by-pass condenser Cv is` provided across the battery I6.

The radio-frequency voltage uctuations, which lare developed across R2, are impressed upon the oscillation circuit bythe feed-back circuit which includes vthe connection through the condenser Cs to thelow'er end'of the inductance L4 and the blocking condenser C4. The inductances La and L4 are inductively coupled, thus providing an electromgnetic' coupling between the feed-back circuit of. the oscillator tube I 'I yand the oscillation circuit. The condenser C4 is common both to the feed-back and oscillation circuits and, therefore, may provide additional coupling between these two circuits.

That portion of the oscillation voltage which is impressed across the condenser C2 will be impressed upon the input of the modulator or first detector tube I5 and thus be combined in the input of said tube with received radio-frequency signal voltages which are also built up across the .condenser C2. The bias of the tube I5 is so arranged that it will act asa modulator, and an audio-modulated intermediate frequency will be produced in the output circuit of the modulator tube. f

The output circuit of the modulator tube I5 includes the radio-frequency choke L5 and the high-voltage source I6. The screen grid of the modulator tube I5-may be supplied with the appropriate voltage by means of a tap in the battery I6. The high-voltage source I6 is by-pessed for highV frequencies by means of condener C7. That portion of battery I6 which supplies the screen grid potential may be similarly provided with a. by-pass condenser Ca.

The intermediate-frequency voltage developed across the choke L5 is coupled through condenser Cn 'to the input of the intermediate-frequency responsive radio receiver 20 by which it is further amplified, detected and reproduced in the usual manner by the loudspeaker 2| The details of the remainder of the receiver and reproducer are common to those found in the art and, therefore, need no further description.

'I'he oscillation-frequency voltages impressed across the inductance L2 may be transferred capacitively, or inductively to the antenna primary inductance L1. Therefore, to prevent radiation of the oscillation-frequency voltages, a neutralization arrangement may be provided as shown. This system comprises the connection between the lower end of the feed-back coil L4 through the neutralizing condenser C1 to the upper end of X Y, s the antenna primary inductance L1. The neutral- The oscillation circuit I8 comprises in series izing capacity C1 is ,so adjusted that the feedback of the oscillation-frequency voltage to the upper end of the coil L1 is just sufficient to 'neutralize that transferred thereto by the tuning inductance Ia.

Appropriate cathode heating circuits may be provided and these, together with' the voltage source represented by the battery I6, may be the vsaine as those provided for supplying the various ductance L3 through this range of frequencies beinductance La. The impedance of the inductance Le, which is in shunt with the condenser C2 across quency voltages that vit will have a negligible effect on these voltages.v

' Thus, -an arrangement has been produced in which the single condenser C2 may be utilized to tune4 two inductances to considerably different frequencies, and the voltages of the two frequen- V cies which are developed across the single variable condenser may be impressed upon the input of the modulator tube I5 and there combined to produce the modulated intermediate frequency in the usual manner. y

The condenserCs acts, as has been stated above, to limit the range of the condenser C2 and thus acts as an alignment condenser by means of which the diierence between the frequency of the signal currents and the frequency of the oscillation currents may be maintained substantially uniform as the condenser Ca is varied to tune the input circuit throughout theA tuning range. The padding condenser Cp permits a three point adjustment of alignment to be obtained. The padding condenser, Cp, isadjusted to correct the alignment when tuned to the upper end of the broadcast band. Theinductance La is adjusted togive correct alignment in the middle of the band, and the condenser C3 is adjusted to correct the alignment at the low-frequency end of the band. These three adjustments are mutually dependent, but when properly made the circuits will maintain their alignment 4as the condenser C2 is tuned over its range. Although condenser C4 is a blocking condenser and has such large capacity as to affect but slightly the tuning of the oscillation circuit, its capacity may be so chosen as to assist in maintaining the alignment, as well as to assist in maintaining uniforml oscillation voltages.

Although the various elements of the circuitv sl-.own may have a wide variety of characteristics, the following approximate values have been found to be satisfactory in producing anv oscillator andv modulator system which' will operate to re-` ceive` signals in the broadcast band lextending from 550 kilocycles to 1500 kilocycles using an intermediate frequency of 4800 kilocycles:

inductance L1=1 mh. inductance La=.2 mh. inductance L3=.015 mh. Inductance L4=.015 mh. Condenser Ca=20 mmf .-500 mmf. Condenser C3=30 mmf. l Condenser C4 .1 mf.Y Condenser Ce=.001 mf. Condenser Cp=10 mmf. Resistor R`a=50,000 ohms. Resistor R4=20,000 ohms.

Although any appropriatel tubes may be used in the circuit described above, a type 58 has been found appropriate for use as the modulator I5,

and a type 56 has been found-to be appropriate and the modulator tube I5. This circuit comprisesthe usual radio-frequency ytuned circuit 1 which is inductively coupled to the inductance L1 kof the antenna circuit and connected in the amplifier is provided between the antenna neutralizing condenser Ci of Fig. 2 is` obviated, as

the oscillator-frequency voltagesimpressed across the inductanceLz will be effectively yblocked by the unilateral transmission characteristics of the screen grid tube 8. The tube 8' may for instance be a pentode, as shown, although any other suitable tube'may be used. It-is obvious, of course, that the input of the tube 8 might be untuned, in 'which event the tube would constitute a coupling` tube. The condenser of the tuned circuit 1 may be connected as shown, to be operated in a unicontrol manner with the condenser Cz of the radio-frequency tuned circuit I4.'

The output of the modulator is impressed upon the input4 of the intermediate frequency amplifier vof the receiverv20 by means of the tunedl intermediate frequency transformer I9, the primary L5 of which: isincludedin the plate circuit of the tube I5.

Appropriate voltage sources may be provided" a similar frequency-changing circuit in which a single tube performs the function of oscillator and modulator, corresponding parts being designated by the same reference characters. In

the circuit shown, the tube I5' is ,of the virtual cathode electron coupled type. Any other type of oscillator-modulator may be used if desired,

` As shown,lthe tuned input circuit Il is connected to number four grid, and this grid, the plate, and a virtual cathode, positioned just inside the number three grid, act as an electron-coupled modulator. Number two grid acts as the oscillator anode, and is connected through resistor Rz to the voltage source "I6. The second` grid circuit comprises the condenser Ce, inductance L4, and condensers C4 and C5. This circuit is coupled to the oscillation circuit I8 bymeans of the inductive coupling between inductance L4 and inductance La and by the capacitive coupling of the condenser C4 which is common to these circuits. The oscillationl voltage is impressed upon the flrstfgrid, to 'which it is directly connected. In operation,.the first grid voltage controis the cathode Vemission, and the lsignal voltage impressed on vthe fourth grid controls the portion of this emission which reaches the plate. This voltage, however, is powerless to affect the voltage of the first grid. The operation is otherwise similar to that of the circuits shown in Figs. 2 and 3.

Whereas the invention has beenfdescribedas a frequency changer for a superheterodyne radio receiver, it may be utilized as the frequency changer for any heterodyne reception system, and it is equally applicable to frequency changers for use in connection with the production of 4'radiant energy'for any purpose, or to any system in which it is desired to tune two resonant'circuits over a range in frequency.. maintaining a constantgfrequency interval between them and using only one variablereactance element.

1. A frequency-chanoine circuit which includes a modulator tube and an oscillator tube, each of said tubes having input and output terminals, a first t'uned circuit connected to the input ter- 'minals of one of said tubes and tunable to a desired frequency, said circuit comprising an inductance and a variable condenser, a second tuned circuit connected across the input terminals of the other of said tubes and tunable to a different frequency, said last-mentioned circuit including a second inductance, a iixed'condenser and. said first-mentioned variable condenser and having reactive constants so proportioned with respect to those of said first circuit that each of said circuits has a high impedance to currents of the frequency to which the other is resonant,

and a coupling between the output of said scillator tube and the tuned circuit connected to the input thereof, whereby the resonant fre quency of each of the tuned circuits is controlled ductance and a variable condenser, a second tuned circuit connected across the input terminais of the other of said tubes and tunable to a different frequency.,l said circuit including a second inductance, a fixed condenser and said first-mentioned variable condenser and having reactive constants so proportioned with respect to those of said first circuit that each 0f said circuits has a high impedance to currents of the frequency to which the other is resonant, a coupling between the output of said oscillator tube and the tuned circuit connected to the input thereof for producing oscillations of a frequency determined by said circuit, and means for neutralizing the oscillation-frequency voltage developed in said input circuit, whereby the resonant frequency of said circuits is controlled by a single variable condenser to tune each of said circuits through a band in frequency.

3. A frequency-changing circuit which includes a modulator tube and an .oscillator tube, each of said tubes having input and output terminals, a first tuned circuit co-nnected to the input terminals of one of said tubes and tunable to a desired frequency, said circuit comprising an inductance and a variable condenser, a second tuned circuit connected across the input terminais of the other of said tubes and tunable to a different frequency, said second circuit including a second inductance, a fixed condenser, and said first-mentioned variable condenser and having reactive constants. so proportioned with respect to those of said iirst circuit that each of said circuits has a high impedance tol currents of the frequency to which the other is resonant,

and a dual coupling between the output of said oscillator tube and the tuned circuit connected to the input thereof, whereby the resonant frequency of each of said tuned circuits is controlled by a single variable condenser to tune said circuits through a band in frequency.

4. A frequency-changing circuit which includes a modulator tube and an oscillator tube, each of said tubes.`having input and output terminals, a first tuned circuit connected to the input terminals of one of said tubes and-tunable to a desired frequency, said circuit comprising an inductance and a variable condenser, a second tuned'circuit connected across the input terminals of the other of said tubes and tunable to a different frequency, said second circuit including a second inductance,y a. xed condenser, and said first-mentioned variable condenser and having reactive constants so proportioned with respect to those of said first circuit that each of said circuits has a high impedance to currents of the frequency to which the other is resonant, and a combined electromagnetic and electrostatic coupling between the output of said oscillator tube and the tuned circuit connected to the input thereof, whereby the resonant frequency' of each of said tuned circuits is controlled by said variable condenser to tune said circuitsl pedance to currents of the frequency to whichl the other is resonant, said second inductance and said fixed condenser being so proportioned that a variation of the capacity of said variable condenser will tune both of said circuits throughout a range in frequency and maintain a substantially constant frequency diiference vbetween the frequencies of each of said circuits, whereby the frequencies of both of said circuits may be simultaneously controlled by a variation of the capacity of said variable condenser.

6. A radio-frequency circuit comprising a first tuned circuit, tunable over a desired range of radio frequencies and comprising an inductance and a variable condenser, and a second tuned circuit, tunable over` a different but substantially equal range of radio frequencies and including a second inductance. a iixed condenser, and said first-'mentioned variable condenser, said second inductance and said fixed condenser being so proportioned thateach of the circuits will have a high impedance to currents of the frequency to which the other is resonant, whereby both of said circuits may be simultaneously tuned by variation of the capacity of said variable condenser.

,7. A radio-frequency circuit comprising a rst tuned circuit, tunable over a desired range of radio frequencies and comprising an inductance and a variable condenser, and a second tuned circuit, tunable over a different range of radio frequencies and including a second inductance, a fixed condenser, and said first-mentioned variable condenser, said second` inductance and said fixed condenser being so proportioned that each of the circuits will have a high impedance to currents of the frequency to which the other is resonant, and will resonate to a frequency having a definite substantially xed rel-ation to the frequency to which the other circuit resonates as both of said circuits are simultaneously tuned through a band in frequency by a variation of the capacity of said variable condenser.

8. A radio-frequency circuit comprising a'iirst tuned circuit, tunable over a desired range of radio frequencies and comprising an inductance and a variable condenser, and a second tuned circuit, tunable over a different range of radio frequencies and including a second inductance, a

fixed condenser and said first-mentioned variable condenser. said second inductance and said fixed condenser being so proportioned that each of the circuits will have a high impedance to'currents of the frequency to which the other is resonant andI will resonate to a frequency having a substantially constant frequency difference relative to the frequency to which the other circuit resonates as both of said circuits are simultaneously tuned through a band in frequency by a variation of the rcapacity of said variable condenser.

9. A radio-frequency circuit lcomprising a first tuned circuit, tunable over a desired range of radio-frequencies and comprising an inductance and a variable condenser. and a second tuned circuit, tunable over a different but substantially equal range of radio frequencies and including a second inductance, a fixed condenser and said first-mentioned variable condenser. said -fixed condenser being so chosen that it will act as a high impedance to currents of the frequency of. the first tuned circuit. and the elementsv of said second circuit being sochosen as to cause said circuit tol be resonant to a frequency to which the first-mentioned inductance has a high impedanQQ. whereby each of said circuits will E resonate at a distinct frequency without affecting the voltages in the other circuit.

10. A frequency-changing circuit which com-l prises a first tuned circuit, tunable to the frequency of a current which it is desired tomodulateand comprising an inductance and a variable Y. condenser, a sccondftuned circuit tunable to the frequency of a modulating current, and including a second inductance. a fixed condenser, and

current it'is desired to modulate, and comprising an inductance and a variable condenser, a second tuned circuit tunable to the frequency of a modulating current,l and including a second inductn ance, a fixed condcnser, and said first-mentioned variable condenser, a modulator tube including input terminals connected across said variable s condenser, means for supplying current of one of said frequencies to said first tuned circuit. and means for supplying an oscillation-frequency voltage vto said second tuned circuit, whereby voltages of both of said frequencies'may be irn-l pressed upon the input of said modulator tube by said variable condenser, said condenser serving 'simultaneously to control the resonance frev quency of both of said circuits.

12. A frequency-changing circuit comprising a t first tunedcircuit, tunable to a desired signal frequency and comprising an inductance and a variable condenser, Va second tuned circuit, tunable to an oscillation frequency and including a sec- Aond inductance, a fixed condenser. and said firstmentioned variable condensc'r, a modulator' tum having its input terminals connected, across said variablefcondenser, an oscillator tube including input and output terminals, .said input terminals being connected across the inductance of said second tuned circuit. and a feed-back circuit connected to the output terminals of said oscillator tube, whereby. voltages of said signal frequency and said oscillation frequency are impressed upon the inputterminals ofsaid modulator tube by said variable condenser, which vcondenser serves to simultaneously control the resonance frequency of each of said circuits. s

13. 'A frequency-changing circuit comprising a nrst tuned circuit, tunable to the frequency oi'` a v desired current itis desired to modulate and comprising an inductance and a variable condenser. a second tuned circuit, tunable to an oscillation frequency and ,includinga second inductance. a

fixed condenser. and said first-mentioned variable condenser, said second inductance and said fixed condenser being so proportioned that a variation of the capacity of said variable condenser will tune both of said circuits through a range in fren quency and maintain a substantially constant difference between the frequencies of said circuits, and a modulator tube having its input terminalscoupled to both of said circuits. whereby voltages of the frequencyI of both of said circuits are impressed upon the input of said modulator tube.

14. A frequency-changing circuit which comprises a first tuned circuit. tunable to a desired radio frequency and comprising an inductance and a variable condenser, a second tuned circuit, tunable to an oscillation frequency and including a second inductance. a fixed condenser. and said first-mentioned variable condenser, said second inductance and said fixed condenser being so proportioned that a variation of thc capacity oi' said variable condenser will tune both of said circuits throughout a range in frequency and maintain a substantially constant frequency dif ference between the frequencies of the current in each of said circuits. means for producing an oscillation-frequency voltage in the second tuned circuit, and a modulator tube having its input terminals connected across said variable condenser, whereby voltages of two frequencies, as controlled by variation of the capacity of said variable condenser, are impressed upon the input of said modulator tubel 15. A frequency-changing circuit which cornprises a first tuned circuit tunable to a desired signal frequency and comprising an inductance and a variable condenser, a second tuned circuit, tunable to an oscillation frequency and including 4a second inductance, a fixed condenser and said inductancepand a feed-back circuit connected be.

tween the output terminals of said oscillator tube 'and coupled to said second tuned circuit for prcducing oscillation frequency currents in said second tuned circuitfwhereby there is produced in the output of said modulator tube a signal-modulated intermediate frequency having a substantially constant frequency as said variable condenser is tuned to receive signals covering a band tunable to a radio frequency, said circuit including an inductance and a variable condenser,'

a. second tuned circuit connected in shunt with said variable condenser and including in series a rst fixed condenser, a second inductance, and a second xed condenser.' said fixed condensers and said second inductance being so proportioned that said second tuned circuit will resonate at a constant frequency difference from the frequency of the first tuned circuit as said variable condenser is varied to tune both o/f'said circuits through a range in frequency, asecond tube includingfinput and output terminals, connections from one end of said second inductance and the junction of said variable condenser and said second fixed condenser to the input terminals of said second tube, and a feed-back circuit connected to the output terminals of said second tubef'or providing a dual feed-back coupling to said second tuned circuit, said feed-back circuit including in series, a feed-back condenser, athird inductance, inductively coupled to said second inductance, and said second fixed condenser, whereby voltages of the frequency of both of said tuned circuits, as controlled by said variable condenser,

I 'are impressed upon the input of said first-menpled to said second inductance, and said second tioned tube.

17. A frequency-changing circuit which includes a modulator tube having input and output terminals, a signal-frequency circuit connected across said input terminals 'and including an xin` ductance tunable by a variable condenser to the frequency of the current to be modulated, an oscillation circuit connected in shunt with said variable condenser and including in series a first fixed condenser, a second inductance, and a second fixed condenser, said fixed condensers and said second inductance being so proportioned that said oscillation circuit will resonate at a constant frequency difference from the frequency of the signal-frequency circuit as said variable condenser is variedto tune said signal-frequency circuit' throughout a range in frequency, an oscil lator tube including input and output terminals, connections from one end of said second inductance and the junction of said variable condenser and said second fixed condenser to said input terminals, and afeedback circuit connected to the output terminals of said oscillator tube for providing a dual coupling to said oscillation circuit, said circuit including in series a feed-back condenser, a third inductance,v inductively couxed condenser, whereby voltages of the frequency of a signal current and an. oscillation current, the frequencies of both of which are determined by said variable condenser, are impressed across said variable condenser and upon the input of said modulator tube, and whereby a signal-modulated difference frequency voltage is produced 'in the output circuit of said moduf lator tube.

- 18. In a frequency-changing circuit, tuned cir- (cuits tunable to the frequency of a signal-fre' restrict passage of currents of the frequency of g andes-1. the input terminals of said nrst vacuum tube and L one of said tuned circuits into the other o f said circuits, and the inductance of one of said tuned circuits being so proportioned as to act as a radio-frequency choke with respect to the frequency produced in the other of said tuned circuits.

19. A radio-frequency circuit comprising a first Vtuned circuit tunable to a desired radio frequency and comprising an inductance and a variable condenser, a second tuned circuit tunable to a different radio frequency, and including a second inductance, a fixed condenser, and said `first-mentioned variablelconden'ser, two thermionic vacuum -tubes each having a pair of input terminals, a connection from one of said tuned circuits to the input terminals of one of said vacuum tubes, and a connection from the second tuned circuit to the input terminals of the other of said vacuum tubes, whereby the frequencies of the currents impressed upon the input of each of said vacuum tubes may be simultaneously varied in the same sense through diilerent` frequency bands by a variation of the capacity of said variable condenser.

20. A frequency-changing circuit which includes a first tuned circuit comprising an in` ductance anda variable condenser, a lsecond tuned circuit including a second inductance, a fixed condenser and said first-mentioned variable condenser and having reactive constants so proportioned with respect to those of said first circuit that each of said circits has a high impedance to currents of the frequency towhich the otherj is resonant, two vacuum tubes each comprising input and output terminals, a con- 1 nection between the first of said circuits and the input terminals of one of said vacuum tubes, a connection between the other of said circuits and the input terminals of the other of said vacuum tubes, and a coupling between the output terminals of one of said vacuum tubes and the input thereof, whereby said tube and its associated circuits operate as an oscillator and modulate currents impressed upon the input terminals of the other of said tubes. A

21. A radio-frequency circuit comprising a first circuit tunable over a desired range of radio frequencies, a second circuit tunable over a substantially equal range of substantially different radio frequencies and having reactive constants so proportioned with respect to those of said first circuit that each of said circuits has a high impedance to currents of the frequency to which the other is resonant, and an adjustable reactanceelement common to said tunable circuits and effective to tune them in the same sense over their respective ranges.

22. A radio-frequency circuit comprising a flrst circuit tunable over a desired range of radio frequencies, a second circuit tunable over a substantially equal range displaced with respect to said rst-named range in the frequency scale and having reactive constants so proportioned with respect to those of said first circuit that each o said circuits has a high impedance to curren l of the frequency to which the other is resonant, and an adjustable condenser common to said tunable circuits and effective to tune them in the same sense over their respective ranges.

23. A radio-frequency coupling network including a plurality" of reactance elements, said network comprising component resonant portions having points of impedance maxima substanv tially displaced on the frequency scale, means for substantially eliminating reactionpf said component portions upon each other. and means for .tuningl said network over a desired range and for adjusting said points in thesame sense and to equal extents comprising lan adjustable reactance element common yto said portions.

24. A radio-frequency coupling network includ-4 ing an adjustable reactance element common to` said portions.

25. A radio-frequency network comprising a first component resonant circuit. a second component resonant .circuit mum impedance displaced on the frequency scale with respect to that of said for substantially eliminatingl reaction of said component circuits upon each other, and means for simultaneously tuning said circuits over a range of frequencies and adjusting'their points of maximum impedance in the same direction and to .equal extents on the frequency scale-comprising an adjustable reactance elementcommon to said circuits.

26. A radio-frequency coupling system comprising a first resonant circuit, a second resonant 4 circuit having reactive constants so proportioned with respectto those of said first circuit that each of said circuits has a high impedance to currents of the frequency to which the other is resonant, a circuit for supplying radio-frequency energy to said circuits,"said circuits including impedance elements individual thereto across which appear voltage maxima displaced on the frequency scale,

for tuning said nethaving apoint of maxi-y first circuit.l means means for simultaneously tuning said circuits over a -range of frequencies and adjusting said points of maximum voltage in the same direction comprising an adjustablev on the frequency scale reactance element commonto said circuits', and output circuits coupled to said impedance elements. x v' f L 4 27. A radio-frequency circuit comprising a first circuit tunable over a desired range of radio frequencies, a second circuit tunable over la range of substantially different radio frequencies "'and 'having reactive constants so proportioned with respect to those of said ilrstcircuit -that each of said circuits has a high impedance to currents of the frequency towhich the other is resonant, an adjustable reactance element common to said tunable circuits and effective to tune them over. their respective ranges, electron-discharge means having two input circuits and an output circuit, said tunable circuits being individually connected to said input circuits, and a coupling between said output circuitland one. of said input fcircuits, whereby said electron-discharge means' and its associated circuits operate as an oscillator and modulate currents impressed upon the other of said input circuits.

` 28. A radiofrequency circuit tunable over a frequencies, a second circuit tunable over a range of substantially different radio frequencies and having reactive constantssol proportioned with respect to those of said first circuit that each of said circuits has a high impedance to currents,

ofthe frequency to which thelgother is resonant, and an adjustable reactanceelement directly connected in circuit with, and common to, both of said tunable circuits and effective to tune them inthe same sense over their respective ranges.-

' DANIEL E. HAnNE'r'r.

circuit comprising a first 1 desired range of radio` 

